RFR : 8231753: use more Posix functionality in aix os::print_os_info

Hello, please review the following small (mostly AIX related) change . It replaces the AIX coding in function void os::print_os_info(outputStream* st) { ... } for uname and load average info output by os::Posix functionality. Additionally it slightly changes os::Posix::print_load_average function to handle the return value of os::loadavg ( indicating failure ) . Bug / webrev : https://bugs.openjdk.java.net/browse/JDK-8231753 http://cr.openjdk.java.net/~mbaesken/webrevs/8231753.0/ Thanks, Matthias